In a first, Stanford scientists have succeeded in restoring key aspects of vision in mice with conditions resembling glaucoma, a disorder that is the second- leading cause of blindness and currently has no cure.
The scientists coaxed optic-nerve cables, responsible for conveying visual information from the eye to the brain, into regenerating after they had been completely severed, and found that they could retrace their former routes and re-establish connections with the appropriate parts of the brain.
That unprecedented, if partial, restoration could pave the way to future work that enables blind people to see.
The animals' condition prior to the scientists' efforts to regrow the eye-to brain-connections resembled glaucoma, the second-leading cause of blindness after cataracts.
Cataracts can often be surgically removed, but there's no cure for glaucoma, said the study's senior author, Andrew Huberman, from Stanford University School of Medicine.
Glaucoma, caused by excessive pressure on the optic nerve, affects nearly 70 million people worldwide. Vision loss due to optic-nerve damage can also accrue from injuries, retinal detachment, pituitary tumours, various brain cancers and other sources.
The retina, a thin sheet of cells no more than half as thick as a credit card, is the light-sensing part of the eye.
Photoreceptor cells in the back of the retina react to different wavelengths of light by sending electrically coded information to other cells in the retina called retinal ganglion cells, of which there are as many as 30 types, each specialising in processing a particular aspect of vision, such as upward motion, motion in general or the colour red.
The ganglion cells project long, electric-wire-like processes called axons, which extend down the optic nerve in a bundle and then fan out to numerous regions of the brain, where they connect with other nerve cells to inform them about the visual world.
Retinal ganglion cells are the only nerve cells connecting the eye to the brain, said Huberman.
When axons in the brain and spinal cord of a mammal such as a mouse or a human have been damaged, they don't regenerate on their own.
The retina, too, is actually part of the brain, said Huberman. Damage to mammalian retinal ganglion cells' axons spells permanent vision loss.
In the study, adult mice in which the optic nerve in one eye had been crushed were treated with either a regimen of intensive daily exposure to high-contrast visual stimulation, in the form of constant images of a moving black-and-white grid, or biochemical manipulations that kicked the mTOR pathway - growth-enhancing molecular interactions - within their retinal ganglion cells back into high gear, or both.
When the two approaches were combined substantial numbers of axons grew and migrated to their appropriate destinations in the brain. Tests of the mice's vision indicated that their once-blind eye could now see.
The study was published in the journal Nature Neuroscience.
